1. Field of the Invention
The present invention relates to a method and apparatus for generating a harmonic overtone by multiplying a frequency of an audio signal. The present invention further relates to a method and apparatus for enhancing a low-frequency component of the audio signal using the generated harmonic overtone.
2. Description of the Related Art
In audio systems such as mini-component stereo systems and flat-screen TV receivers, small-diameter speakers are used, and enclosures (i.e., speaker boxes) accommodating speakers are also small in volume. Such speakers have a high lowest reproducible frequency f0 of about 100 Hz or more.
In general, when a low-frequency component of not greater than the lowest reproducible frequency f0 is supplied to the speakers, as the frequency decreases, the output sound pressure level of a fundamental-wave component decreases and the number of distortion components (harmonic-wave components) rapidly increases.
In audio systems including such small-diameter speakers, it is difficult to sufficiently reproduce low-frequency sounds of not greater than the lowest reproducible frequency f0 of the speakers.
Therefore, a technique based on characteristics of the human perception to generate the impression of low-frequency sounds has been conceived. For example, the sound of a musical instrument is composed of a fundamental tone and harmonic overtones thereof, and the timbre or tone color of the musical instrument is determined by the fundamental-to-overtone ratio. Psycho-acoustically, the human auditory system allows for perception of a fundamental tone being output if harmonic overtones thereof are output even though no fundamental tone is actually being output.
Japanese Unexamined Patent Application Publication No. 8-213862 discloses the use of this feature. That is, an audio signal is separated into a low-frequency component and a high-frequency component. The low-frequency component is alternately written in first and second buffers at predetermined time intervals, and is alternately read from the first and second buffers at intervals of a predetermined time by a thinning-out method. The frequency of the low-frequency component is multiplied by a factor of “a” (e.g., a factor of two). The resulting signal after the multiplication is combined with the high-frequency component using a combining unit.
The above publication only shows circuit structures and frequency characteristics but does not show a waveform chart or time chart. FIG. 11 shows a harmonic-overtone generation method of the related art based on thinning-out reading, which is disclosed in the above publication.
A low-frequency component Slin is a signal component in an audio signal, having a frequency not greater than a lowest reproducible frequency of a speaker (in the above publication, the lowest reproducible frequency is referred to as a “resonant frequency”). Although represented by an analog waveform in FIG. 11, the low-frequency component Slin is digital data including data of samples.
In the harmonic-overtone generation method of the related art disclosed in the above publication, the low-frequency component Slin is divided into segments with constant periods T10, T20, T30, etc., each corresponding to a predetermined number of samples. The samples of the low-frequency component Slin are alternately written in first and second buffers at intervals of a fixed time such that the samples corresponding to the period T10 are written in the first buffer and the samples corresponding to the period T20 are written in the second buffer.
In the read operation, the same samples are repeatedly read twice at intervals of a fixed time alternately from the first and second buffers. That is, in a first half period T21 of the period T10, the samples written in the first buffer within the period T10 are read from the first buffer in a ratio (or proportion) in which one sample is thinned out and one sample is extracted for every two samples. Also in a second half period T22 of the period T20, the samples written in the first buffer within the period T10 are read from the first buffer in a ratio in which one sample is thinned out and one sample is extracted for every two samples. In a first half period T31 of the period T30, the samples written in the second buffer within the period T20 are read from the second buffer in a ratio in which one sample is thinned out and one sample is extracted for every two samples. Also in a second half period T32 of the period T30, the samples written in the second buffer within the period T20 are read from the second buffer in a ratio in which one sample is thinned out and one sample is extracted for every two samples.
Therefore, as shown in FIG. 11, a harmonic-overtone signal Slout having a frequency twice that of the low-frequency component Slin is obtained as an output signal.
The harmonic-overtone signal Slout is combined with a high-frequency component of the input audio signal to obtain a low-frequency-enhanced output audio signal. As described above, the impression of low-frequency sounds is generated.